The goal of this project (recently transferred from LVD to LPD with three personnel) is to evaluate a variety of factors which influence the virulence of pathogenic protozoa, especialy Entamoeba histolytica and Trichomonas vaginalis. Ongoing studies continue evaluating amebal viruses, selection pressures by repeated animal passage at limiting dilution, and an amebal "toxin" in the virulence of several strains of E. histolytica. Amebal extracts are toxic for mammalian cells, producing rounding and aggregation of cells in tissue culture, closely resembling the cytotoxicity of the plant lectin, concanavalin A. The former effect is inhibited by normal serum of many species. We have identified the serum component as the glycoprotein, fetuin. Fetuin and the hexosamine, N-acetyl-D-galactoamine inhibit and reverse the cytotoxicity of amebal extracts as does alpha-methyl-mannoside the cytotoxicity of concanavalin A. We believe the amebal toxin to have lectin-like properties. We are employing affinity chromatography to purify the "toxin" for further study.